1) A measurement, used for a variety of commodities.
1419 Custus vitri: Et in iij sem’ et in iij pais’ albi vitri emptis de Johanne Glasman ... prec. le seme, 20s, York
1456-7 pro ij seym salmon de Joh’e Neusom xvs, Fountains Abbey. It is found in charcoal accounts as equivalent to a horse-load, one of three alternative terms for the same quantity: 1567-8 the quarter ys a seame and the seme ys 8 busshell
the quarter ys a horsselode
the dusson ys 12 seme or 12 quarter alle one mesuer, Esholt.
2) In coal-mining, veins in the rock.
This is a word already in use in the Old English period when it referred to the seam formed when the edges of two pieces of cloth were joined by sewing. A much later example indicates how it came to be used of rock strata: 1687 ‘that which at first seemed to be seams or joynings of the Stones are only veins in the rock’. Coal-mining examples date from that period.